1. Field of the Invention
The present invention relates generally to superabrasive cutting elements, and more specifically to polycrystalline diamond compact cutting elements, comprised substantially of diamond optionally bonded to a reduced-mass supporting substrate.
2. State of the Art
Fixed-cutter rotary drag bits have been employed in subterranean drilling for many decades, and various sizes, shapes and patterns of natural and synthetic diamonds have been used on drag bit crowns as cutting elements. Rotary drag-type drill bits are typically comprised of a bit body having a shank for connection to a drill string and encompassing an inner channel for supplying drilling fluid to the face of the bit through nozzles or other apertures. Drag bits may be cast and/or machined from metal, typically steel, or may be formed of a powder metal (typically WC) infiltrated at high temperatures with a liquified (typically copper-based) binder material to form a matrix. It is also contemplated that such bits may be formed with so-called layered manufacturing technology, as disclosed in U.S. Pat. No. 5,433,280, assigned to the assignee of the present invention and incorporated herein by this reference.
The bit body typically carries a plurality of cutting elements mounted directly on the bit body or on a carrier element. Cutting elements may be secured to the bit by preliminary bonding to a carrier element, such as a stud, post, or cylinder, which in turn is inserted into a pocket, sachet, recess or other aperture in the face of the bit and mechanically or metallurgically secured thereto. Polycrystalline diamond compact (PDC) cutting elements may be brazed directly to a matrix-type bit or to a pre-placed carrier element after furnacing, or even bonded into the bit body during the furnacing process. It has also been suggested that PDC cutting elements may be adhesively bonded to the bit face or to a carrier element.
For over a decade, it has been possible to process diamond particles into larger disc shapes. The discs, or diamond tables, are typically formed of sintered polycrystalline diamond, the resulting structure being freestanding or bonded to a tungsten carbide layer during formation. A typical PDC diamond table/WC substrate cutting element structure is formed by placing a disc-shaped cemented carbide substrate including a metal binder such as cobalt into a container or cartridge of an ultra-high pressure press with a layer of diamond crystals or grains loaded into the cartridge adjacent one face of the substrate. A number of such cartridges are typically loaded into a press. The substrates and adjacent diamond crystal layers are then compressed under ultra-high temperature and pressure conditions. These conditions cause the metal binder from the substrate body to become liquid and sweep from the region behind the substrate face next to the diamond layer through the diamond grains to form the polycrystalline diamond structure. As a result, the diamond grains become mutually bonded to form a diamond table over the substrate face which is bonded to the substrate face. The spaces in the diamond table between the diamond-to-diamond bonds are filled with residual metal binder. It is also possible to form freestanding (no substrate) polycrystalline or monocrystalline diamond structures, providing another source of binder is employed, as is known in the art. For example, powdered binder may be intermixed with the diamond grains.
A so-called thermally stable PDC product (commonly termed a TSP) may be formed by leaching out the metal in the diamond table. Alternatively, silicon, which possesses a coefficient of thermal expansion similar to that of diamond, may be used to bond diamond particles to produce an Si-bonded TSP. TSPs are capable of enduring higher temperatures (on the order of 1200.degree. C.) without degradation in comparison to normal PDCs, which experience thermal degradation upon exposure to temperatures of about 750-800.degree. C. TSPs are typically freestanding (e.g., without a substrate), but may be formed on a substrate. TSPs may also be coated with a single- or multi-layer metal coating to enhance bonding of the TSP to a matrix-body bit face.
Any substrate incorporated in the cutting element must sufficiently support the diamond table to curtail bending of the diamond or other superabrasive table attributable to the loading of the cutting element by the formation. Any measurable bending may cause fracture or even delamination of the diamond table from the substrate. It is believed that such degradation of the cutting element is due at least in part to lack of sufficient stiffness of the cutting element so that, when encountering the formation, the diamond table actually flexes due to lack of sufficient rigidity or stiffness. As diamond has an extremely low strain rate to failure, only a small amount of flex can initiate fracture.
PDC cutting elements, with their large diamond tables (usually of circular, semi-circular or tombstone shape), have provided drag bit designers with a wide variety of potential cutter deployments and orientations, crown configurations, nozzle placements and other design alternatives not previously possible with the smaller natural diamond and polyhedral, unbacked synthetic diamonds (usually TSPs) traditionally employed in drag bits. These PDC cutting elements, with their large diamond tables extending in two dimensions substantially transverse to the direction of cut have, with various bit designs, achieved outstanding advances in drilling efficiency and rate of penetration (ROP) when employed in soft to medium hardness formations, and the larger cutter dimensions and attendant greater protrusion or extension above the bit crown have afforded the opportunity for greatly improved bit hydraulics for cutter lubrication and cooling and formation debris removal.
Since the early days of PDC use on drill bits, however, it has been apparent that PDCs suffer thermal degradation at the high temperatures generated by the frictional abrasive contact of the PDC cutting edge with the formation as the bit rotates and weight is applied to the drill string on which the bit is mounted. Such degradation leads to premature dulling of the PDC cutting edge, and even gross failure of the PDC cutting element assembly. Improved feedstock and fabrication techniques have raised the thermal tolerance of PDCs to some degree. As noted above, there has been developed a subcategory of PDCs known as thermally stable products, or TSPs, which retain their physical integrity to temperatures approaching 1200.degree. C. TSPs may be infiltrated into matrix body drill bits at the time of bit furnacing, rather than being attached at a later time, as with non-thermally stable PDCs. However, even TSPs suffer from thermal degradation during cutting of the formation as the drill bit advances the wellbore.
While the prior art has focused on problems associated with the degradation of the diamond layer or table, heating of the cutting element substrate (typically tungsten carbide) from the drilling operation is also detrimental to cutting element performance. Heat checking of the substrate, typically caused in one form by alternative heating and quenching of the cutting elements as the drill bit bounces on the bottom of the borehole and drilling fluid intermittently contacts the cutting elements at the cutting edges, can initiate more severe substrate cracking which, in turn, can propagate cracking of the diamond table.
A variety of attempts have been made to cool and clean PDC cutting elements during the drill operation by flushing the cutting elements with drilling fluid, or "mud," pumped down the drill string and through nozzles or other orifices on the face of the bit. The flow of drilling mud removes formation cuttings and other debris from the face of the bit and generally radially outwardly to the bit gage, up the junk slots and into the wellbore annulus between the drill string and the wall of the wellbore to the surface, where the debris is removed, the mud screened and reconditioned with additives and again pumped down the drill string. It is known in the art to direct drilling mud flow across the face of a series of cutting elements (U.S. Pat. No. 4,452,324 to Jurgens); to direct mud flow from a nozzle toward the face of a single cutting element (U.S. Pat. No. 4,303,136 to Ball); and to direct flow from a nozzle to a single cutting element at a specific orientation (U.S. Pat. No. 4,913,244 to Trujillo). It has also been proposed to direct mud flow through the face of a PDC cutting element from an internal passage extending from the interior of the drill bit through the carrier element and out an aperture in the face of the cutting element (U.S. Pat. No. 4,606,418 to Thompson).
It has also been proposed, in U.S. Pat. No. 4,852,671 to Southland, to direct drilling mud flow through a passage in a stud supporting a PDC to a relief between the pair of cutting points in the formation-contacting zone of a disc-shaped PDC cutting element to improve the cooling and cleaning of the cutting elements. Moreover, in U.S. Pat. No. 5,316,095 to Tibbitts, the cutting element is cooled with drilling fluid via a plurality of internal channels having outlets adjacent the peripheral cutting edge of the diamond cutting element.
In addition to degradation of the cutting element due to thermal effects, the interface of the diamond table with the substrate (typically tungsten carbide, or WC) is subject to high residual shear stresses arising from formation of the cutting element, as after cooling, the differing bulk moduli and coefficients of thermal expansion of the diamond and substrate material result in thermally-induced stresses. In addition, finite element analysis (FEA) has demonstrated that high tensile stresses exist in a localized region in the outer cylindrical substrate surface and internally in the WC substrate. Both of these phenomena are deleterious to the life of the cutting element during drilling operations, as the stresses, when augmented by stresses attributable to the loading of the cutting element by the formation, may cause spalling, fracture or even delamination of the diamond table from the substrate.
In addition to the foregoing shortcomings, state of the art PDCs often lack sufficient diamond volume to cut highly abrasive formations, as the thickness of the diamond table is limited due to the inability of a relatively thick diamond table to adequately bond to the substrate. Further, as the diamond table wears in the prior art cutting elements, more and more of the substrate material becomes exposed to the formation, increasing the so-called "wear flat" area behind the cutting edge of the diamond table and resulting in less-efficient cutting for a given weight on bit (WOB). Moreover, the frictional coefficient of diamond in contact with rock is much lower than that of the substrate material. Thus, as the wear flat increases, friction and frictionally-induced heating of the cutting element increase.